FREE eBOOK

Astrophotography

The Lunar 100

Many Lunar 100 selections are plainly visible in this image of the full Moon, while others require a more detailed view, different illumination, or favorable libration. North is up.

S&T: Gary Seronik

Just about every telescope user is familiar with French comet hunter Charles Messier's catalog of fuzzy objects. Messier's 18th-century listing of 109 galaxies, clusters, and nebulae contains some of the largest, brightest, and most visually interesting deep-sky treasures visible from the Northern Hemisphere. Little wonder that observing all the M objects is regarded as a virtual rite of passage for amateur astronomers.

But the night sky offers an object that is larger, brighter, and more visually captivating than anything on Messier's list: the Moon. Yet many backyard astronomers never go beyond the astro-tourist stage to acquire the knowledge and understanding necessary to really appreciate what they're looking at, and how magnificent and amazing it truly is. Perhaps this is because after they identify a few of the Moon's most conspicuous features, many amateurs don't know where to look next.

The Lunar 100 list is an attempt to provide Moon lovers with something akin to what deep-sky observers enjoy with the Messier catalog: a selection of telescopic sights to ignite interest and enhance understanding. Presented here is a selection of the Moon's 100 most interesting regions, craters, basins, mountains, rilles, and domes. I challenge observers to find and observe them all and, more important, to consider what each feature tells us about lunar and Earth history.

Anatomy of the Lunar 100

Objects in the Lunar 100 are arranged from the easiest to view to the most difficult. This is more systematic than the haphazard approach that produced the Messier list. Indeed, just by knowing a feature's Lunar 100 number, you have some idea of how easy or challenging it will be to see. For example, the Moon itself is L1, while L2 is earthshine and L3 is the light/dark dichotomy between lunar highlands and maria ("seas"). I'd be surprised if anyone reading this couldn't tick those off the list right now. Higher-numbered objects are smaller, less conspicuous, or positioned closer to the limb, making them more challenging to locate and view.

Planetary scientist Charles Wood's Lunar 100 is a list of telescopic sights designed to ignite interest in the Moon and enhance understanding of its geology.

Source: Antonín Rükl

The Messier objects are scattered all over the sky, but all are theoretically observable during marathon nights in March and April every year. By contrast, the Lunar 100 are concentrated in just ½°of sky, yet they can't all be seen in a single night, or even in a single month. Some lunar objects can be observed only with grazing solar illumination, while others are albedo features that require full-Moon conditions to be seen. And others are positioned near (or sometimes even over) the limb of the Moon, requiring a very favorable libration to bring them into view. I don't know how quickly all 100 can be observed, but I'm sure that some competitive amateur will complete it faster than I dare guess!

How big a telescope do you need to view the Lunar 100? The smallest features listed are 3 kilometers in diameter and thus nominally visible in 3-inch (75-millimeter) telescopes employing magnifications of about 150× to 200×. And many can be found with smaller scopes at lower power. But a few Lunar 100 objects — such as narrow rilles — are best seen with 6- or 8-inch telescopes used at high power. The goal, however, is not just to find the objects, but to understand what they tell us abut the Moon.

Any selection of lunar features is bound to lead to many difficult judgments, and I'm sure that at least a few of my choices and rankings will generate considerable debate. Some of my choices were obvious, some were not. Some were influenced by my personal sense of what crater appears more dramatic than another, or which rille best demonstrates an aspect of the Moon's evolution. Aesthetics aside, my choices were principally governed by a desire to include features that tell us something important or interesting about the Moon itself.

I invite you to use the Lunar 100 to guide your explorations of the Moon.

The Lunar 100

L

Feature Name

Significance

Lat. (°)

Long. (°)

Diam. (km)

Rükl Chart

1

Moon

Large satellite

—

—

3,476

—

2

Earthshine

Twice reflected sunlight

—

—

—

—

3

Mare/highland dichotomy

Two materials with distinct compositions

—

—

—

—

4

Apennines

Imbrium basin rim

18.9N

3.7W

70

22

5

Copernicus

Archetypal large complex crater

9.7N

20.1W

93

31

6

Tycho

Large rayed crater with impact melts

43.4S

11.1W

85

64

7

Altai Scarp

Nectaris basin rim

24.3S

22.6E

425

57

8

Theophilus, Cyrillus, Catharina

Crater sequence illustrating stages of degradation

13.2S

24.0E

—

46, 57

9

Clavius

Lacks basin features in spite of its size

58.8S

14.1W

225

72

10

Mare Crisium

Mare contained in large circular basin

18.0N

59.0E

540

26, 27, 37, 38

11

Aristarchus

Very bright crater with dark bands on its walls

23.7N

47.4W

40

18

12

Proclus

Oblique-impact rays

16.1N

46.8E

28

26

13

Gassendi

Floor-fractured crater

17.6S

40.1W

101

52

14

Sinus Iridum

Very large crater with missing rim

45.0N

32.0W

260

10

15

Straight Wall

Best example of a lunar fault

21.8S

7.8W

110

54

16

Petavius

Crater with domed & fractured floor

25.1S

60.4E

177

59

17

Schröter's Valley

Giant sinuous rille

26.2N

50.8W

168

18

18

Mare Serenitatis dark edges

Distinct mare areas with different compositions

17.8N

23.0E

N/A

24

19

Alpine Valley

Lunar graben

49.0N

3.0E

165

4

20

Posidonius

Floor-fractured crater

31.8N

29.9E

95

14

The Lunar 100 (continued)

L

Feature Name

Significance

Lat. (°)

Long. (°)

Diam. (km)

Rükl Chart

21

Fracastorius

Crater with subsided & fractured floor

21.5S

33.2E

124

58

22

Aristarchus Plateau

Mysterious uplifted region mantled with pyroclastics

26.0N

51.0W

150

18

23

Pico

Isolated Imbrium basin-ring fragment

45.7N

8.9W

25

11

24

Hyginus Rille

Rille containing rimless collapse pits

7.4N

7.8E

220

34

25

Messier & Messier A

Oblique ricochet-impact pair

1.9S

47.6E

11

48

26

Mare Frigoris

Arcuate mare of uncertain origin

56.0N

1.4E

1600

2–6

27

Archimedes

Large crater lacking central peak

29.7N

4.0W

83

12, 22

28

Hipparchus

First drawing of a single crater

5.5S

4.8E

150

44, 45

29

Ariadaeus Rille

Long, linear graben

6.4N

14.0E

250

34

30

Schiller

Possible oblique impact

51.9S

39.0W

180

71

31

Taruntius

Young floor-fractured crater

5.6N

46.5E

56

37

32

Arago Alpha & Beta

Volcanic domes

6.2N

21.4E

26

35

33

Serpentine Ridge

Basin inner-ring segment

27.3N

25.3E

155

24

34

Lacus Mortis

Strange crater with rille & ridge

45.0N

27.2E

152

14

35

Triesnecker Rilles

Rille family

4.3N

4.6E

215

33

36

Grimaldi basin

A small two-ring basin

5.5S

68.3W

440

39

37

Bailly

Barely discernible basin

66.5S

69.1W

303

71

38

Sabine & Ritter

Possible twin impacts

1.7N

19.7E

30

35

39

Schickard

Crater floor with Orientale basin ejecta stripe

44.3S

55.3W

227

62

40

Janssen Rille

Rare example of a highland rille

45.4S

39.3E

190

67, 68

The Lunar 100 (continued)

L

Feature Name

Significance

Lat. (°)

Long. (°)

Diam. (km)

Rükl Chart

41

Bessel ray

Ray of uncertain origin near Bessel

21.8N

17.9E

N/A

24

42

Marius Hills

Complex of volcanic domes & hills

12.5N

54.0W

125

28, 29

43

Wargentin

A crater filled to the rim with lava or ejecta

49.6S

60.2W

84

70

44

Mersenius

Domed floor cut by secondary craters

21.5S

49.2W

84

51

45

Maurolycus

Region of saturation cratering

42.0S

14.0E

114

66

46

Regiomontanus central peak

Possible volcanic peak

28.0S

0.6W

124

55

47

Alphonsus dark spots

Dark-halo eruptions on crater floor

13.7S

3.2W

119

44

48

Cauchy region

Fault, rilles, & domes

10.5N

38.0E

130

36

49

Gruithuisen Delta & Gamma

Volcanic domes formed with viscous lavas

36.3N

40.0W

20

9

50

Cayley Plains

Light, smooth plains of uncertain origin

4.0N

15.1E

14

34

51

Davy crater chain

Result of comet-fragment impacts

11.1S

6.6W

50

43

52

Crüger

Possible volcanic caldera

16.7S

66.8W

45

50

53

Lamont

Possible buried basin

4.4N

23.7E

106

35

54

Hippalus Rilles

Rilles concentric to Humorum basin

24.5S

29.0W

240

52, 53

55

Baco

Unusually smooth crater floor & surrounding plains

51.0S

19.1E

69

74

56

Australe basin

A partially flooded ancient basin

49.8S

84.5E

880

76

57

Reiner Gamma

Conspicuous swirl & magnetic anomaly

7.7N

59.2W

70

28

58

Rheita Valley

Basin secondary-crater chain

42.5S

51.5E

445

68

59

Schiller-Zucchius basin

Badly degraded overlooked basin

56.0S

45.0W

335

70, 71

60

Kies Pi

Volcanic dome

26.9S

24.2W

45

53

The Lunar 100 (continued)

L

Feature Name

Significance

Lat. (°)

Long. (°)

Diam. (km)

Rükl Chart

61

Mösting A

Simple crater close to center of lunar near side

3.2S

5.2W

13

43

62

Rümker

Large volcanic dome

40.8N

58.1W

70

8

63

Imbrium sculpture

Basin ejecta near & overlying Boscovich & Julius Caesar

11.0N

12.0E

—

34

64

Descartes

Apollo 16 landing site; putative region of highland volcanism

11.7S

15.7E

48

45

65

Hortensius domes

Dome field north of Hortensius

7.6N

27.9W

10

30

66

Hadley Rille

Lava channel near Apollo 15 landing site

25.0N

3.0E

—

22

67

Fra Mauro formation

Apollo 14 landing site on Imbrium ejecta

3.6S

17.5W

—

42

68

Flamsteed P

Proposed young volcanic crater & Surveyor 1 landing site

3.0S

44.0W

112

40

69

Copernicus secondary craters

Rays & craterlets near Pytheas

19.6N

19.1W

4

20

70

Humboldtianum basin

Multi-ring impact basin

57.0N

80.0E

650

7

71

Sulpicius Gallus dark mantle

Ash eruptions northwest of crater

19.6N

11.6E

12

23

72

Atlas dark-halo craters

Explosive volcanic pits on the floor of Atlas

46.7N

44.4E

87

15

73

Smythii basin

Difficult-to-observe basin scarp & mare

2.0S

87.0E

740

38, 49

74

Copernicus H

Dark-halo impact crater

6.9N

18.3W

5

31

75

Ptolemaeus B

Saucerlike depression on the floor of Ptolemaeus

8.0S

0.8W

16

44

76

W. Bond

Large crater degraded by Imbrium ejecta

65.3N

3.7E

158

4

77

Sirsalis Rille

Procellarum basin radial rilles

15.7S

61.7W

425

39, 50

78

Lambert R

A buried "ghost" crater

23.8N

20.6W

54

20

79

Sinus Aestuum

Eastern dark-mantle volcanic deposit

12.0N

3.5W

90

33

80

Orientale basin

Youngest large impact basin

19.0S

95.0W

930

50

The Lunar 100 (continued)

L

Feature Name

Significance

Lat. (°)

Long. (°)

Diam. (km)

Rükl Chart

81

Hesiodus A

Concentric crater

30.1S

17.0W

15

54

82

Linné

Small crater once thought to have disappeared

27.7N

11.8E

2.4

23

83

Plato craterlets

Crater pits at limits of detection

51.6N

9.4W

101

3, 4

84

Pitatus

Crater with concentric rilles

29.8S

13.5W

97

54

85

Langrenus rays

Aged ray system

8.9S

60.9E

132

49

86

Prinz Rilles

Rille system near the crater Prinz

27.0N

43.0W

46

19

87

Humboldt

Crater with central peaks & dark spots

27.0S

80.9E

207

60

88

Peary

Difficult-to-observe polar crater

88.6N

33.0E

74

4, II

89

Valentine Dome

Volcanic dome

30.5N

10.1E

30

13

90

Armstrong, Aldrin & Collins

Small craters near the Apollo 11 landing site

1.3N

23.7E

3

35

91

De Gasparis Rilles

Area with many rilles

25.9S

50.7W

30

51

92

Gylden Valley

Part of the Imbrium radial sculpture

5.1S

0.7E

47

44

93

Dionysius rays

Unusual & rare dark rays

2.8N

17.3E

18

35

94

Drygalski

Large south-pole region crater

79.3S

84.9W

162

72, VI

95

Procellarum basin

The Moon's biggest basin?

23.0N

15.0W

3200

—

96

Leibnitz Mountains

Rim of South Pole-Aitken basin

85.0S

30.0E

—

73, V

97

Inghirami Valley

Orientale basin ejecta

44.0S

73.0W

140

61

98

Imbrium lava flows

Mare lava-flow boundaries

32.8N

22.0W

—

10

99

Ina

D-shaped young volcanic caldera

18.6N

5.3E

3

22

100

Mare Marginis swirls

Possible magnetic field deposits

18.5N

88.0E

—

27, III

Chart numbers refer to Antonín Rükl's Atlas of the Moon.

For the convenience of observers, the Lunar 100 is also available on Sky & Telescope's 9-by-12-inch laminated Lunar 100 Card ($6.95), featuring a high-quality Moon map by Antonin Rukl on the front. The reverse side shows the locations and sizes of 100 features, together with brief descriptions of each.